The need for improved illumination sources used for characterization of ever-shrinking integrated circuit device features continues to grow. Semiconductor metrology and inspection systems require very stable, very bright (high radiance) broadband light sources to perform precise measurements of small dimensions and/or detect small defects. Increasing the brightness of light sources creates higher throughput and higher sensitivity.
In previous approaches, Xe, Ag or Hg arc lamps have been used to produce broadband light. The arc lamps include an anode and cathode, which generate an electric discharge to excite and ionize the gas and sustain it at a high temperature, while broadband light is emitted from the excited and ionized gas. During operation, the anode and the cathode become very hot, and are prone to wear by evaporation and sputtering of material from their surfaces. Material lost from the electrodes can contaminate the gas and envelope and reduce its light output (particularly at UV wavelengths, where even a very thin layer of material deposited on the lamp envelope or window can substantially reduce UV transmission) or result in failure of the light source. More importantly, these arc lamps do not provide sufficient brightness (spectral radiance) for some applications, including inspection and metrology applications within the semiconductor and related industries. The brightness of arc lamps is limited by the attainable current density, which in turn is limited, in part, by the need to avoid excessive wear of the electrodes and an uneconomically short lamp lifetime.
Spectral radiance, or brightness (i.e., the emitted light power per unit area per unit solid angle per unit wavelength), is very important for light sources intended for use in semiconductor inspection and metrology systems. Such systems typically illuminate a relatively small area at any one time (such as an area with dimensions between a few microns and a few hundred microns). The light used to inspect or measure a sample needs to be focused into this small area on the sample with sufficient power to produce enough reflected and/or scattered light to create a signal with a high signal-to-noise ratio. Since an optical system comprising lenses, mirrors etc. can, at best, only preserve spectral radiance (if completely lossless), a high spectral radiance is required from the light source to deliver a high power into a small area. It is noted that, at best, simply increasing the power and size of the plasma of a plasma lamp will provide an inefficient means to increase the amount of power delivered to a given area, and, at worst, may not increase the power that can be delivered to the given area at all.
Arc lamps simply lack sufficient brightness for critical inspection and metrology applications in the semiconductor industry. The lifetime is limited due to the hot temperature of the electrodes. Furthermore, the position of the arc can be unstable.
In some inspection and metrology systems, a laser-sustained (LSP) plasma lamp has been implemented. A LSP lamp can be brighter than an arc lamp, emit over a larger spectral range and have a much longer lifetime. A LSP lamp may comprise a transparent envelope (such as an envelope made from fused silica) with two electrodes and filled with pressurized gas similar to a conventional arc lamp. A laser beam at an infra-red (IR) wavelength may be focused to the center of the plasma. A brief electrical discharge is created between the electrodes by applying a high voltage to ignite a plasma and hot gas where the laser is focused. The laser energy absorbed by the plasma and hot gas is used to sustain the plasma after the voltage between the electrodes is turned off. The tightly focused laser can generate a plasma size as small as 100 microns and a plasma temperature between 10,000K and 20,000K. Because of the small size and high temperature of the plasma compared with a conventional arc lamp (which typically has an arc length of a few mm), LSP light sources are much brighter and emit more light with short wavelengths. Since an electrical discharge between the electrodes exists only briefly to start the lamp, wear of the electrodes is dramatically reduced or made negligible, greatly increasing the lamp life compared with a conventional arc lamp. Furthermore, the size of the plasma is a better match to the source size required by typical semiconductor inspection and metrology systems so that the collection efficiency can be higher compared to a conventional arc lamp.
While LSP lamps are brighter than the arc lamps, in order to meet the demand for inspecting/measuring ever smaller defects, existing LSP light sources are insufficient. Simply increasing the laser pump power merely increases the size of the plasma and the surrounding hot gas, while the center of the plasma does not become significantly hotter. This occurs because the most of the laser pump light power is absorbed by the hot, but largely unionized gas, surrounding the plasma, resulting in little of the increased pump power reaching the plasma core. As a result, the brightness of a LSP plasma source tends to saturate at high pump powers. In addition, as the pump laser power increases, the plasma can become unstable.
Therefore, it would be desirable to provide a broadband source that cures the various shortcomings of prior approaches, such as those described above.